The present invention relates to a method for automatic cleaning of a cylinder, especially a cylinder of a printing machine, in which the cylinder is provided with plates and used for printing on a print carrier made of e.g. paper, plastic film, or metal film, and in which the cylinders become smudged, during printing, with printing ink, particles detached from said print carrier such as dust or fibres, and other foreign objects, wherein an area of the cylinder surface is cleaned by exposing said area to a pressurized flow of fluid so that the foreign objects inside the area on the cylinder surface are detached, and wherein said area is exposed to a vacuum to remove said detached foreign objects and any other material originating from said flow of fluid.
So-called flexographic printing on a print carrier of paper, plastic film or metal film uses a system consisting of a first cylinder, around which a web of the print carrier runs, a second cylinder performing the actual printing and being provided with plates for this purpose, and a third cylinder transmitting ink to the second cylinder.
When the printing process has continued for a certain time, there will normally arise problems with the printing quality. This is because the second cylinder, the printing cylinder, has become smudged with dust, fibres or other particles from the web of paper, plastic film or metal film. When this occurs, it is necessary to halt the printing process, displace the cylinder from its bearing and then manually brush or wash impurities off the second cylinder. This is a serious inconvenience since it means that the stoppage periods may amount to as much as 30 per cent of the total time in which the machine is in use. This means that the operation time in which printing takes place may risk being reduced to 70 per cent of the time in which the machine is in use. Out of those 30 per cent of time in which the machine is halted, up to 90 per cent is due to cleaning the printing cylinder. Furthermore, the waste of paper, plastic film or metal film is substantial because there is a certain running-in period after a stoppage and the web of paper or film being printed during this period does not have a sufficient quality and must be discarded.
EP-0,369,565 describes a cleaner for automatic cleaning of a cylinder by pressing or otherwise discarding impurities from the surface of the cylinder. The cylinder cleaner comprises a brush extending along the total length of the cylinder or, in another embodiment, a bar for applying pressurized air or ultrasound. The cleaning takes place by rotating or displacing the brush along the entire length of the cylinder or by applying pressurized air or ultrasound along the entire length of the bar. This cleaner, however, has the great inconvenience that the efficiency of the cleaning is strongly reduced. Firstly, cleaning by using a brush quickly results in the brush being worn up together with the surface of the cylinder being worn. Secondly, cleaning by using pressurized air or ultrasound being applied to the total length of the cylinder has the effect that the efficiency of the cleaning is not the same over the total length of the cylinder. The pressurized air or the ultrasound is applied at one end of the bar and, therefore, the efficiency of the cleaning will be highest at the first end of the bar and lowest at a second end of the bar opposite the end at which the pressurized air or the ultrasound is applied.
WO 94/12349, a document published after the priority of the present invention, describes an apparatus also for cleaning the surface of a cylinder. The apparatus comprises a jet nozzle being located inside a suction nozzle. The jet nozzle emits a single jet of liquid, and the liquid together with impuries detached from the surface of the cylinder are evacuated through the suction nozzle. The jet nozzle is arranged in the centre of the suction nozzle, and at the opening of the suction nozzle a means for supplying compressed air into the chamber of the suction nozzle is also provided. This apparatus has the inconvenience of using a jet of liquid for cleaning the surface of the cylinder. After cleaning, it will be necessary to wait until the surface is completely dry before further printing can take place. However, an even more serious inconvenience of the apparatus is that only one jet nozzle is provided. Due to this effect the cleaning process will take a large amount of time because the single and relatively small jet of liquid has to be displaced along the entire length of the roller in order to clean the whole surface of the roller. This displacement, therefore, has to take place very slowly in order to ensure cleaning of the whole surface.
JP 63-4947 describes af further apparatus for cleaning the surface of a roller in a typographic rotary press. This apparatus comprises a jet nozzle for supplying a jet of pressurized air, contrary to liquid, and also a vacuum duct for evacuating the impurities detached from the surface of the roller. Like in the previous publications, the jet nozzle is located inside in the vacuum duct. Although using pressurized air, this apparatus also has the great inconvenience of having only one jet nozzle for supplying the pressurized air. Like before, the jet nozzle has to be displaced along the length of the roller very slowly in order to ensure that the whole surface of the roller is cleaned. Therefore, the cleaning process when using the apparatus described in that publication will also take a very large mount of time.
Thus, it is the object of the present invention to provide a method which may be carded out automatically, and in which the above-mentioned inconveniences, such as unsuitable, cleaning media and very long time for carrying out the cleaning process, are avoided.
This object is achieved by a method characterized in that the cylinder surface is constituted by adjacent areas lengthwise and crosswise on the cylinder surface provided by such area extending over a minor part of the circumference of said cylinder and a minor part of the length of said cylinder, and that said adjacent areas of said cylinder surface are cleaned successively, wherein the pressurized fluid is provided by mixing pressurized air and a liquid in a mixing chamber, and that the so-formed mixture is led to the print plate through nozzles being connected with the mixing chamber.
In addition, apparatuses for use by the method will be disclosed.
By the method according to the present invention it is now possible, during operation, to clean a cylinder, particularly a flexographic printing cylinder. In this manner one avoids the substantial time waste previously associated with cleaning the cylinder. Reducing the time waste, one also reduces the waste of material accordingly as cleaning is accomplished during ordinary operational conditions. If cylinder cleaning is carried out before it is so smudged as to deteriorate the printing quality, there will be neither material nor time waste during cleaning.
By using several nozzles or a slot, the area around the length of the cylinder is extended as compared to using only one nozzle. Thereby the amount of time used for cleaning the surface is strongly reduced. A larger area, although still just a small area of the surface, is cleaned and accordingly, an orifice with a cleaning can be displaced along the length of the roller at a much higher speed. As mentioned before, it is very important to reduce the amount of time used during the cleaning process.
The method is advantageous in that cleaning is performed automatically and preferably while the printing process is running.
As pressurized air containing the admixed liquid is used for detaching the particles from the printing cylinder it is possible to obtain a secure and effective cleaning even if ink and dust stick very hard to the printing cylinder. This will not be possible with the liquid jets nor with the air jets. Surprisingly, and in contradiction to the expectation for the skilled in this art it has shown that is is possible to use such mixture in a cleaning process while the printing process is running. It has shown that the cleaning will not harm the quality of the printing even though it is not a dry cleaning as explained in the JP 63-4947. It is believed that the reason for the effective cleaning is that a mist is produced, which mist is led to the printing cylinder. Such mist may be sucked away very effective in comparison with a liquid even if rather limited level of vakuum is used.
For various reasons, however, it may be advantageous to use other media, e.g. depending on the ink used for printing, the material being printed on, the kind of particles to detach from the cylinder, or the speed at which the printing process is operating. Among other options for cleaning media besides a mixture of pressurized air and liquids, there could be mentioned a media influenced by an ultrasound field, and likewise various kinds of solid matter particles may be added to the fluid media constituting a fluidized medium.
Having become detached from the cylinder surface, the particles will normally have to be removed from the surface. This is accomplished in that, after the particles have been detached from the surface, the cylinder is exposed to a vacuum sucking the particles off the surface. Any other material originating from the cleaning process, e.g. solid matter particles which have been used in the cleaning process, may be sucked off at the same time. It will also be possible to load the particles that are to be detached, or have become detached, with static electricity and subsequently to use an electric voltage field to remove the particles from the cylinder surface.
In order to ensure that the cylinder with the plates is cleaned both on and between the plates, the cleaning medium should preferably be directed in an inclined angle in relation to the tangent of the cylinder surface. This will ensure that particles depositing on the sides of the plates are removed as well. Thus, the inclined flow will attack both the cylinder surface and the cylinder sides in an inclined angle, not parallel or perpendicular. The method is further advantageous in that no damage is done to the plates during the cleaning process.
As mentioned, the method is suitable for cleaning printing cylinders in flexography which printing process uses cylinders with printing plates. However, the method may be used for many types of rollers and cylinders, not just printing cylinders.
Apparatuses for use by the method according to the present invention may be designed in many different ways. Two types of apparatuses are disclosed according to the present invention. One apparatus comprises mobile cleaning members in the shape of a cleaning head being slid over the cylinder surface whereby successive cleaning of the surface takes place. The other apparatus comprises fixed cleaning members provided with an internal device likewise conducting a successive surface cleaning. It is a common feature that an antechamber is provided for effecting a mixing of liquid and a pressurized air.